With developments of mobile phone technology and car navigation system technology, sound processing technology, including speech recognition, speaker recognition and voice synthesis, has been widely used in many fields. There is a technique for determining a mental state (feeling) of a speaker by analyzing a voice sound uttered by the speaker. This technique is to detect characteristics such as a “strained voice” and a “hoarse voice” in an interval of a frequency signal on the basis of a frequency fluctuation of the waveform of the sound output from a sound source and a fluctuation of a high frequency component of the sound, select an acoustic model suitable for the characteristics and provided for speech recognition, and determine the mental state of the speaker on the basis of the acoustic model and the sound source to which the voice sound is input. In addition, there is a technique for analyzing a mental state of a speaker on the basis of the average of sound volumes in certain frames and a dispersion of the sound volumes. The aforementioned techniques are disclosed in Japanese Patent No. 4085130 and Japanese Laid-open Patent Publication No. 2004-317822, for example.